added xml schemas for describing xml documents that define the
properties needed for the creation of data units. 
this is used to eliminate the core data unit classes and have more
dynamic way of adding new data units (and pdus) to the project.
this approach chases a new software architecture that can be easily
extended which isn't possible with the current approach.
furthermore, the main language will become groovy as it makes it easier
to follow the desired architecture because it's a dynamic typed language
(and I want to learn groovy ;) ).